The present invention relates to a control system for an automobile automatic transmission of the type having a mechanical planetary gear train with hydraulically actuated gear shift elements for shifting of the transmission elements.
In automatic hydrodynamic-mechanical transmissions, the turbine of the hydrodynamic torque convertor is normally connected positively with the drive gears, when one of the forward gears is selected, by a so called forward gear clutch. Even during idle, however, the engine transmits hydraulic energy through the convertor pump to the turbine wheel so that a moment is produced which causes the vehicle to crawl, i.e. to travel slowly forward even though the accelerator pedal is released. Usually, then, in the case of automatic transmissions, the brake must be employed to keep the vehicle stationary or the gear select lever must be moved in the neutral position.
Crawling of the vehicle during idling may also be eliminated by providing a special clutch arranged between the turbine wheel and engine actuated by an electrical idling switch. An additional clutch of this type, however, requires considerable space and is costly.
An improved control system for automatically disengaging the forward gear clutch during certain operating states of the vehicle is shown and described in commonly owned U.S. patent application Ser. No. 61,058, filed July 26, 1979 (corresponding to West German Patent application P 28 33 641.6). A manual control valve, dependent upon the position of the transmission gear select lever, conveys a pressure medium when the lever is in one of the forward drive positions for actuating a gear shift element (forward gear clutch) associated with the gear. A main control valve, dependent on the position of the accelerator pedal, is disposed in the hydraulic fluid line between the manual control valve and the gear shift element. The main control valve controls the supply of fluid from the manual control valve to the selected gear shift element for disengaging the gear shift element if the vehicle is stationary, the accelerator pedal is released, and the gear select lever is in the Drive (D) position.
A second fluid pressure line, connected to the main control valve, acts on the valve to connect the manual control valve and gear shift element. When the accelerator pedal is released, a purging control valve vents the supply of second fluid acting on the main control valve, causing the main control valve to interrupt the supply of hydraulic fluid to the gear shift element and to exhaust the fluid in the gear shift element to disengage the element.
In the case of a vehicle having the above-described control system, should the driver leave the vehicle with the engine running and forget to move the drive-selector lever out of the forward drive position D or engage the hand brake, the vehicle could move unexpectedly. For example, if the accelerator pedal or the throttle-flap lever connected therewith were accidentally touched (such as in the case of an engine test), the forward clutch would re-engage and the vehicle would move forward.